<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<style>
    .d1{
        background-color: aqua;
        width: 100px;
        height: 100px;
    }
    .d2{
        background-color: rgb(255, 94, 0);
        width: 200px;
        height: 200px;
    }
</style>
<body>
    <div class="d1">

    </div>
    <script>
        var div = document.querySelector('div');
        div.onclick = function(){
            div.setAttribute('class','d2');
        }
    </script>
</body>
</html>